Induction motors are among the simplest and most rugged electric motors and are made up of a wound stator and a rotor assembly. The rotor assembly resembles a squirrel cage from one end so that the motor is commonly called a squirrel cage motor. A shaded pole motor is an AC single phase induction motor which has a squirrel cage rotor. Single phase motors require some means for producing a rotating magnetic field for starting the motor. The shaded pole motor, a part of the face of each field pole carries a copper ring called the shading coil which induces a rotating magnetic field.
In many applications, it is desirable to bring an AC motor to a rapid and controlled stop. One technique for accomplishing this in an AC motor is to apply a direct current across the windings of the motor. The application of direct current to the windings generates an electromagnetic force within the motor to stop the rotor with a rapid braking action. In braking systems using a synchronous motor, a starting capacitor in the AC motor may be charged during operation of the motor and then electrically coupled to the windings of the AC motor to produce a braking force on the motor when the motor is switched off. However, sometimes braking affects afforded by the starting capacitor does not stop the motor within the desired time due to the limited charge the starting capacitor can store to increase the braking affects. Some prior art systems utilize a braking capacitor having a greater storage capacity. During the motor's operation, the braking capacitor is charged. When the motor is stopped, the braking capacitor is applied across the windings of the motor to bring the motor to a rapid halt.
The present invention is directed towards an alternating current induction motor which switches from an AC load to applying a DC current to the motor for purposes of holding or locking the rotor in place against torque being placed on the motor shaft and on the rotor and to accomplish a greater holding ability of the rotor with a smaller amount of DC current.
Prior art electric braking systems for alternating current motors can be seen in the Hastings U.S. Pat. No. 5,705,903 for an electric brake circuit for bringing an alternating current motor to a rapid halt which uses a braking capacitor charged to a preselected voltage. In the Oltendorf, U.S. Pat. No. 3,475,669, a variable dynamic direct current brake circuit for an AC motor is provided while in the Gross U.S. Pat. No. 3,872,363 an electric motor braking system is utilized. In the Gross U.S. Pat. No. 3,798,523, a single phase induction motor brake uses a capacitor and a current limiting impedance in a DC braking circuit. The Guttmann, U.S. Pat. No. 4,195,255, is for an electric brake for AC motors which has a control rectifying means for applying direct current to the motor. The Ramirez, Jr. et al., U.S. Pat. No. 6,906,493, provides for an electric brake for a motor by applying a direct current voltage from a capacitor across terminals of an alternating current motor. The Gritter et al., U.S. Pat. No. 4,990,844, is a DC braking of an inverter driven AC motor. In the Nagel, U.S. Pat. No. 4,185,770, an automatic flue damper control system is normally energized to hold the damper closed as a mechanical biased mechanism to move the damper to an open position in the absence of motor energization. A braking arrangement applies a direct current to the motor for dynamic braking as the damper closely approaches its open position.
In contrast to this prior art, the present invention is directed towards a single phase induction motor in which direct current is applied to the terminals of an alternating current motor for purposes of holding the rotor in a stationary position against a biasing torque placed on the rotor. The present rotor holding circuit is accomplished using a reduced amount of electric power.